This application claims the priority of German patent document 10 2008 026 497.0-55, filed Jun. 3, 2008, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating an active lateral-view sensor when its height above the surface to be detected is variable.
Active lateral-view sensors or lateral-view instruments use RADAR (Radio Wave Detection and Ranging) or LIDAR (Light Wave Range and Detection) to measure the interval between transmission and echo return. They are used by satellites, for example, to survey and image the earth's surface. One example of an active lateral-view sensor is the Synthetic Aperture Radar (SAR) System, which is also called lateral-view radar. In such SAR Systems, alternately, i) pulse signals are emitted at defined time intervals by way of a short antenna that is moved in the direction of the surface to be detected, and ii) the echo signals (and thus the pulse signals reflected at the scanned objects) are received. For each region covered and scanned by the antenna, a SAR processor computes an image of the scanned object by a corresponding data processing of the echo signals.
By means of their emitted pulse signals, lateral-view sensors, such as SAR systems cover a region or swath of a surface to be detected or surveyed (or of an atmospheric volume) abreast of the flight direction, and receive echo signals from this region or swath of a surface to be detected or surveyed abreast of the flight direction. The viewing direction of the sensor, (i.e., the direction of the emitted pulse signals, or scanning beams), normally remains the same—for example, according to an orientation relative to the flight direction and nadir direction. Even when various swaths are defined (angle-of-view regions), which can be selected at random (for example, by means of a phased RADAR antenna), the orientation of each swath itself remains the same during the entire orbit of the sensor around the celestial body that is to be surveyed. However, on elliptical paths and around non-spherical-symmetrical celestial bodies, the distance varies between a lateral-view sensor and the surface that is to be detected, as illustrated in FIG. 1.
The time behavior during a recording or scanning therefore depends considerably on the height (altitude) above the measured surface and on the region to be detected. If, for example, an active lateral-view sensor is operated such that, alternately, pulse signals can be emitted and echo signals can be received only during the non transmission intervals, the pulse repetition frequency (PRF) (the frequency of the emission of pulsed signals) must be adapted continuously, which increases considerably the complexity of the control of the scanning. In an access range to be detected, with, for example, an angle of incidence of from 20° to 55° between the scanning beams emitted by a lateral-view sensor and the surface to be surveyed, there are a number of overlapping individually selectable swaths whose orientation with respect to the angle of incidence or angle of view remains similar during the entire orbit. The values of the PRF that are possible based on the signal propagation time vary during the orbit. A fixed PRF for each swath is often not possible.
Therefore, one object of the present invention is to provide a process for optimizing the operation of an active lateral-view sensor when the height above the surface to be detected is variable.
This and other objects and advantages are achieved by the method according to the invention, in which the scanning beams emitted by the lateral-view sensor for scanning a surface to be detected are adjusted as a function of the determined height of the sensor, by means of a roll rotation such that the variation of the surface to be detected is reduced during the orbit of the lateral-view sensor. In other words, the present invention uses a roll steering rule for aligning the antenna of a lateral-view sensor to define a swath of a surface to be detected that is to be covered. As a result, in contrast to a rigid antenna alignment, the lateral-view sensor can be operated at a constant PRF during longer sections in the orbit; and its operation can therefore be optimized with respect to the complexity of the control of the scanning of the surface to be detected. Furthermore, one antenna beam per swath to be scanned of the surface to be detected may be sufficient during the entire orbit of the lateral-view sensor.
According to one embodiment, the invention provides a process for optimizing the operation of an active lateral-view sensor when the height above the surface to be detected is variable, which has the following steps:                Continuous determination of the height of the lateral-view sensor above the surface to be detected, and        adjusting the scanning beams emitted by the lateral-view sensor for scanning the surface to be detected by roll rotating (roll steering rule) as a function of the determined height of the lateral-view sensor, such that a variation of the surface to be detected is reduced during the orbit of the lateral-view sensor. This arrangement simplifies control of the scanning of the surface to be detected by operation with a constant PRF during longer sections of the orbit of the lateral-view sensor as an optimization according to this embodiment of the invention.        
According to an embodiment of the invention, the adjustment by roll rotating may comprise a mechanical rotation of the lateral-view sensor.
In an embodiment of the invention, as an alternative or in addition, the adjustment by roll rotating may comprise an electronic roll rotation of the emitted scanning beams, by a corresponding control of an antenna of the lateral-view sensor.
According to an embodiment of the invention, the adjustment by roll rotation may comprise a swiveling of the direction detected by the lateral-view sensor as a function of the surface to be detected.
According to another embodiment of the invention, the adjustment by roll rotation may comprise maintaining a fixed distance between the lateral-view sensor and the detected surface.
Furthermore, according to an embodiment of the invention, the adjustment by roll rotation may comprise maintaining a fixed width of a swath of the surface to be detected that is covered by the lateral-view sensor.
Furthermore, according to an embodiment of the invention, the adjustment by roll rotation may comprise variation, during the orbit of the lateral-view sensor, of an angle of view that all swaths in the access range covered by the lateral-view sensor have in common.
Furthermore, according to an embodiment of the invention, the orientation of the lateral-view sensor corresponding to its orbital position can be adapted by yawing and pitching, such that a zero Doppler recording geometry is ensured over the entire access range of the lateral-view sensor.
In another embodiment, the invention relates to a system for optimizing the operation of an active lateral-view sensor while the height above the surface to be detected is variable, the system for implementing a process according to the invention being constructed as explained above and having the following:                First devices for the continuously determining the height of the lateral-view sensor above the surface to the detected, and        second devices for adjusting the scanning beams emitted by the lateral view sensor for scanning the surface to be detected by roll rotating such that a variation of the surface to be detected is reduced during the orbit of the lateral-view sensor. The system may, for example, be constructed as a module that can be integrated in a satellite.        
Finally, according to an embodiment, the invention relates to a lateral-view sensor, for example, a SAR system which includes a system according to the invention, as explained above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.